This invention relates to an internal combustion engine and more particularly to an apparatus for preventing the explosive combustion of unburnt gas transferred into the exhaust system after the ignition switch of the engine has been turned off.
Even when the ignition switch is turned off, an engine cannot stop its rotational movement immediately due to inertial rotating forces. Fuel sucked from the carburetor during this inertial rotation period is introduced, still unburnt, through the cylinders into the exhaust system where it produces explosive combustion. This explosive combustion of the unburnt gas in the exhaust system is known as "afterburning".
Recently, air pollution caused by exhaust gases from internal combustion engines has become a serious problem. Increasingly, the trend is to use engines in which the exhaust systems are provided with a reactor that purifies the exhaust emissions by re-combusting hydrocarbons (HC) and carbon monoxide (CO) contained in these emissions. It has been found, however, that afterburning occurs in the reactor, which has frequently led to such serious troubles as damage to or breakdown of the muffler or other parts of the exhaust system.
In order to reduce the output of nitrogen oxides (NOx) that are present in exhaust gases together with other noxious substances, some engines are designed or adjusted to burn lean air-fuel mixtures. But in such lean mixture combustion systems, the air-fuel mixture transferred unburnt to the exhaust system of the engine during the inertial rotation period can also cause explosive combustion. Thus the afterburning phenomenon has been a severe problem for the lean mixture combustion systems as well.
There is a known method to prevent afterburning that continues igniting the mixture during the inertial rotation of the engine when the engine rotates above a predetermined speed after the turning off of the ignition switch. This method, however, has a disadvantage in that it causes the inertial rotation period to increase, and, in the extreme case, the engine may continue to "diesel" for a considerable length of time.